Sound reproducing apparatus



Jan. 10, 1933. c. w. RICE ET AL 7 1,894,197

I v SOUND REPRODUCING APPARATUS Filed Aug. 21, 1931 Invenror's: ChesTerW Rice, Edward W- Kellogg,

Patented Jan. 10,1933

UNITED STATES PATENT OFFICE CHESTER w. RICE, OI SGHENEOTADY, YORK, AND EDWARD W. KELLOGG, OI

EOOBESTO'WN, NEW JERSEY, ASSIGNORS TO GENERAL ELECTRIC COMPANY, A COB- POBATION 01' NEW YORK scum) nnraonqcmearrana'rus Application iled August 21, 1881. Serial No. 558,698.

Our present invention relates to apparatus for producing sound from electric currents corresponding to original sounds, and more particularly to devices now commonly known as loud s eakers.

One 0 the objects of our inventionus to provide a device of the class mentioned whereby the reproduction of sounds may be made with greater accuracy and fidelity than with devices of this class previously employed.

A further object of our inventlon 1s to pro vide an apparatus in which the usual horn may be dispensed with and the sound projected from the device in a more satisfactory manner than with the usual form of horn.

Our present application is a continuation in part of our application Serial No. 1482, filed in the United States Patent Oflice J anuary 9, 1925.

The novel features which we believe to be characteristic of our invention are set forth with particularity in the appended claims. Our invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure is a view partly in cross-section of a device constructed in accordance with our invention.

We have indicated in the drawing a sound reproducing device comprising a diaphragm of conical form which, in the device illustrated, is specifically a truncated cone. This diaphragm in the form shown is made up of two parts. The inner member 1, which may be of thin light metal, such as aluminum, and an outer member 2 of light, fibrous material, such for example, as paper,'which is secured to the inner member 1, by suitable adhesive, or otherwise, as desired. By reason of its conical form the diaphragm may be light and at the same time rigid enough so that it will be capable 'of vibrating substantiall as a whole. A cylindrical member 3, whic is secured to the top of the inner member 1, car ries an actuating coil 4, to which the current for operating the loud speaker, may be supplied. The coil 4 is disposed in the annular air gap 5, which is formed between the oon- Y centric pole pieces 6 and 7. The inner'pole piece 7 is surrounded by a coil 8, through which a steady direct current may be supplied for producing a strong magnetic field in the annular air gap 5. The outer end or base of the diaphragm is supported by means of stretched strings 9, or by other suitable means in an opening 10 of a baflie member 11. The strings 9 permit freedom of movement of the diaphragm in the longitudinal direction, but prevent movement in a lateral direction whereby the edges might strike against the wall of the opening 10. When variable currents are supplied to the coil 4:, the diaphragm will be caused to vibrate substantially as a whole in accordance with the variations in current supplied to the coil. Because of the flexible nature of the supporting strings 9 there may be a tendency of lateral movement in the coil 4 in the air gap 5. To avoid this there is provided a ring of flexible material, for example, such as rubber or thin leather 12, which is secured to the base of the inner member 1, and extends outwardly therefrom to a supporting ring 13. This flexible ring 12 also permits freedom of longitudinal movement of the diaphragm, and the strings 9vand the ring 12, together constitute an efiective means for preventing lateral movement of any part of the diaphra The restoring force of lhe diaphragm mounted in the way illustrated may be made so low that the natural rate of vibration as a whole will be below the lowest important voice frequency. It is possible to construct such a device in a Way that its natural rate of vibration as a whole will be about fifty cycles. Fairly good results however may be obtained with diaphragms having higher natural frequencies of vibration even up to 200 cycles, particularly if provision is made for damping so that the resonance is not sharp. This low natural frequency is an essential feature in the construction of loud speakers, in order that the diaphragm shall It wili be observed therefore that the space between the inner member 1 and the inner pole 7 will be almost completely closed. Suitable venting of this space may be furnished by means of the holes 14 in the rin 3. The centering ring 15, which is provi ed for maintaining the proper spaced relation between the pores 6 and may also be cut away, as indicated at 16, to permit air to more readily circulate between the ring 3 and pole piece 7. With the arran ment as thus far described, air can circulate etween the front and back of the diaphragm around the supporting member 11 without undergoing an appreciable compression. This would resu t in substantial loss of the low tones. If, however, the path between the front and back of the diaphragm is increased to a distance of about one-fourth of a wave length or more, this circulation will be substantially prevented and good radiation will be secured. This effect may be made by making the supporting member 11 of fairly massive or rigid material and making it large enough to effectively prevent such circulation. This bafile effect may conveniently be obtained by inclosing the diaphragm and its actuating mechanism in a cabinet of suitable form. Preferably the back of this cabinet should be open or vented to prevent any air space within the cabinet.

While it may be shown theoretically that a small diaphragm, of such rigid construction that all parts have equal motion even at the highest frequencies, and so supported that its natural frequencyof vibration as a whole is below the lowest frequency to be radiated, constitutes an ideal sound reproducer as explained in Patent 1,795,214, issued to E dward W. Kellogg, on March 3, 1931, we have not found it necessary in practice to adhere to diaphragms of such small size as wouldbe needed to comply with the condition just mentioned. For example, we have made a calculation which indicates that a paper cone, four inches in diameter at the base, free at the outer edges and with an angle of 45 between 7 axis and element will probably begin to depart materially fromv true plunger action at a frequency between 3000 and 4000 vibrations per second. Cones with larger 'diameterspr with larger angles between element and axis will begin to show appreciable phase and amplitude differences between the different parts of their surfaces at lower frequencies.

e have nevertheless obtained good results with cones up to 8 and even 20 inches in diameter and with angles as great as 64, provided that in all cases the outer edge of the cone is so flexibly supported that at low frequencies the cone moves as a whole. The probable explanation of our experience is that at the higher frequencies the diaphragm radiates sound by virtue of a progressive deflection. When a force is applied-at thesmall resonance effect due to confinedv end of the a circular flexural wave travels out toward the outer edge, and gives off sound waves in the air as it pro The progressive deflection we shall re er to as wave action in the diaphragm while the type of motion in which all parts of the surfaces have the same motion ma be described as piston or plunger action. the devices built in accordance with our invention, true plunger action takes place at the lower frequencies and wave action presumably takes at the higher frequencies. The transition is not abrupt, nor is it marked by any great chan in radiation characteristic or efliciency w 'ch would upset the balance between the high and low frequency com nents or impair the general quality of sound roduction. Within the frequency range 0 aracterized by wave action in the diaphragm slight resonance may occur, for the reason that reflection of the waves takes lace at the outer edge of the diaphragm. e fact that resonances do not appear to be as sharp or as strong as to impair sound reproduction is probably due to the high attenuation of the waves at high frequency by energy absorption in the material of'the diaphragm, and also in part to ener absorption in the flexible supports of the 'a hragm.

As compared with 'aphragms so small that we can be sure that they act as plungers throughout the entire essential frequency range, we find a gain in sensitivity from the use of somewhat larger sizes, our best results having been obtained with diameters between four and eight inclrs and angles between element and axis, from 45 to Diaphragms of large diameter work best with the larger angles. In the case of sound reproducing devices employing conic'al diaphragms, either rigidly supported at the outer edge, or with a support whose stiffness in the axial direct on is comparable with the stiffness of the diaphragm 1tself,'plunger action cannot take place at any frequency. Ifa vibratory force is applied at or near tlile verfltfxfthe resultin motion'is greatestw ere e orce is apph ed and becomes less as the edge, is approached if the frequency is below the fundamental resonance. As the frequency israised a series of resonanca occurs. At certain frequencies the motion is greatest at the point of application of the driving force, and at other frequencies the motion is greatest at oints between-the vertex and outer edge.

lexural waves originating at the driving point, travel to the outer edge where they are reflected andproduce standing waves with circular nodes and loops of motion. At low frequencies the resonances are far apart on the musical sca1e,.and owing to the small absorption of energ, resonance is sharp. Both of these facts mean serious distortion in the 'reproducton of sounds. At higher frequen- Ill ' musical scale, and are less sharp because of eater relative energy absorption in the material of the diaphragm and sound radiation is therefore more nearly uniform. With a diaphragm stifliy supported at the edge, there is little motion and'little sound radiation at frequencies below the first resonance. Therefore, to secure anywhere near adequate radiation of low tones it is necessary to make the diameter large and the angle between element and axis large (giving a shallow cone). Our tests indicate, however, that cones which are very shallow in comparison with their diameters fail to give adequate radlation of the very high frequency tones.

A device employing a flexibly supported conical diaphragm has the following advantages over previous devices having rigid or stifi support around the outer edge. The radiation of the low tones is much more nearly uniform and free from resonance, and the radiation of high frequencies is more satisfactory for the reason that we have more latitude in the choice of cone angle and can choose that which ives the most nearly uniform radiation of t e high frequencies. Devices with stifiiy su ported outer edges must employ an angle w 'ch is a compromise between what is best for the high and low frequencies and is therefore not the best angle for either one. Our device has the further advantage that with plunger diaphragm action we can obtain adequate radiation of low tones with a smaller diphragm, provided that we use a bafiie in connection with the diaphragm. This means that the diaphragm is easier to protect from injury and the resultin device becomes more compact.

While in the drawing we have shown a diaphragm which is made up of two members, similar results may be obtained with diaphragms constructed of a single member of paper or other similar light material, with the inner member 1 omitted and the actuating coil 4 secured directly to the top of the diaphragm. It also will be evident that numerous modifications may be made in the manner in which the diaphragm is supported without departing from the scope of our invention as set forth in the appended claims. What we claim as new and desire to secure by Letters Patent of the United States is 1. In an apparatus for sound reproduction,

a diaphragm having the form of a trun :ated

for suspending said diaphragm having so low a restoring force that its natural rate of -vibration as a whole will be below the lowest important voice frequency, means for actuating said diaphragm comprising a pair of V concentric pole pieces separated by an annular air gap, an actuating coil secured to the top of said diaphragm and locatedin said air gap, and means in addition 'to the suspendm .means for preventing lateral movement 0 nular air gap, an actuating coil secured to the top of said diaphra and located in said air gap, and means in a dition to the suspending means for preventing lateral movement of said coil in sa d air fgap 3. In an apparatus or sound reproduction, a diaphragm of conical form having a diameter between four and eight inches, the angle between an element and the axis of said diaphragm being of a value between 45 and and actuating means for producing a vibration thereof, said diaphragm being so suspended and actuated that the restoring force applied thereto will be so low that its natural rate of vibration as a whole will be below the lowest important voice frequency.

4. In an apparatus for sound reproduction, a diaphragm of conical form having a diameter between four and eight inches, the angle between an element and the axis of said dlaphragm being of a value between 45 and 7 0, an actuating means for, producing a vibration thereof, said diaphragm being so suspended and actuated that the restoring force applied thereto will be so low that its natural rate of vibration as a whole will be below 200 cycles.

5. In an apparatus for sound re roduction, a diaphragm of conical form having a diameter between four and eight inches, and which is so sup orted that it is capable of vibrating as a who e the angle between an element an the axis of said dlaphragm being of a value between 45 and 70, and means for applying a driving force to the top of said diaphra m, the restoring force applied to said diaphragm being so low that its natural rate of vibration as a whole will be below the lowest important voice frequency.

6. In an apparatus for sound reproduction, a diaphragm of conical form having a diameter between four and eight inches, and which is so supported that it is capable of vibrating substantially asa whole, the an le between an element and the axis of said diaphragm being of a value between 45 and 7 0,

being I made up of an inner member of metal and an outer member of fibrous material, means for actuating said diaphragm comprising a pair of concentric pole pieces separated by an annular air gap, an actuating coil secured to the top of said inner member and located in said air ap, means attached to the outer member or supportin said diaphragm so flexibly that it is capa le of vibrating substantialiy as a whole, and means attached to the inner member for preventing lateral movement of the actuating coil in said air gap.

8. In an apparatus for sound reproduction, a diaphragm having the form of a truncated cone which is made up of an inner member of metal and an outer member of fibrous material, means for actuating said diaphragm comprising a pair of concentric pole pieces separated by an annular air gap, an actuating coil secured to the top of said inner member and located in said air gap, means attached to the base of the outer member at said diaphragm for flexibly supporting it so that it is free to vibrate substantially as a whole, and means extending outwardly from the base of the inner member for preventing lateral movement of the actuating coil in the air gap.

In witness whereof we have hereto set our hands.

CHESTER RICE. EDWARD W, KELLOGG. 

